Error context for string interpolation may be wrong

If there are multiple replacements like `'%s, %s' % (one, other)`, many errors (e.g. wrong replacement type) are reported with the tuple expression as the location. It would be better to set specific item as the error context as the current way can be quite misleading if one uses, `--show-column-numbers`, or `--pretty`, or one has a multi-line interpolation. 